1. Field of Invention
The present invention belongs to the field of mobile communications technologies, and relates to a cluster head assisted method for converting a user terminal from device-to-device (D2D) communication to cellular communication.
2. Description of Related Arts
D2D cluster communication refers to directly exchanging information among a group of neighboring terminals through direct links, which does not require conventional network (for example, a base station) transfer. As the terminals are mobile, the cluster communication technology needs to provide mobile support for the terminals, and the most fundamental support is mobile switching/conversion between D2D communication and cellular communication. For D2D cluster communication, mobile management is especially important because apart from mobile switching, there are also issues about cluster update and re-establishment, for example, a terminal joins a cluster or leaves a cluster. As a favorable supplement to regular wireless communications, D2D cluster communication does not need network transfer. The network herein includes any access point such as a base transceiver station (BTS), a NodeB, an eNodeB, and a site controller.
D2D cluster communication has two operating modes: one is cluster communication under centralized control of a base station, and the other is cluster communication under distributed control of a cluster head. In a communication process of the first operating mode, the base station controls and manages D2D cluster communication, and each D2D member keeps a cellular link connection with the base station to complete exchange of control signaling, and each member terminal completes data exchange through a D2D link. In a communication process of the second operating mode, the cluster head controls and manages intra-cluster signaling and data exchange of the cluster communication; in this mode, the cluster head and the base station keep a cellular link connection in a communication connected state, and exchange necessary control signaling such as synchronization signaling, access signaling, and resource allocation (re-allocation) signaling; a member terminal in the cluster keeps a D2D link connection with the cluster head, is managed and controlled by the cluster head, and has no cellular link with the base station. However, due to the mobility of the terminal, the cluster communication cannot continue unless it is updated, reestablished, or even switched to a cellular network. This is no doubt a great challenge to distributed and semi-distributed cluster communication.
As shown in FIG. 1, in D2D pair communication under centralized control of a base station, once the quality of a D2D link fails to meet a communication requirement, UE1 and UE2 in the D2D communication mode are converted to a cellular communication mode by means of the base station. The process is as follows: initially, UE1 and UE2 directly communicate with each other through a D2D link. UE1 and UE2 periodically detect and measure the quality of the D2D link; once the link quality is lower than a predetermined threshold (THRD1) within a period of time, UE1 and UE2 each send a request to the base station, hoping to be converted to the cellular communication mode. After receiving the switching request, the base station allocates wireless resources to UE1 and UE2, and then the base station forwards a response message to UE1 and UE2. Subsequently, UE1 and UE2 separately measure and calculate quality of a cellular link, and if the quality of the cellular link is higher than a predetermined threshold (THRD2), both UE1 and UE2 request releasing D2D resources. After that, the base station sends a D2D resource release response message to UE1 and UE2. Finally, UE1 and UE2 release the D2D resources, and are converted to the cellular communication mode. At this time, the switch process from D2D communication to cellular communication is finished. In this switch process, it is assumed that two D2D terminals belong to the same base station, and each D2D terminal is capable of exchanging control signaling with the base station. However, if it is D2D cluster communication involving a group of user terminals rather than D2D pair communication, this method consumes a large quantity of wireless resources and much control signaling. Moreover, this method does not apply to D2D cluster communication under distributed control either.
FIG. 2 shows a simplified process of S1-based handover procedure in a Long Term Evolution (LTE) system. First, if there is no X2 interface between two base stations, or a source base station has initiated switching to a specific base station through an S1 interface, S1 switching will be initiated. Then, the source base station sends switching request information to a source mobility management entity (MME) (Step 2), and the source MME forwards a relocation request to a target MME (Step 3). Then the target MME sends a “switching request” to a target base station (Step 4), and the target base station establishes resources (Step 5), and feeds back a “switching request response” to the target MME (Step 6); the target MME responds the relocation request to the source MME (Step 7). After that, the source MME sends a “switching command” to the source base station (Step 8), and the source base station forwards the switching command to the terminal (Step 9). Subsequently, the source base station sends “base station state transfer” to the source MME (Step 10). After the target base station receives the “MME state transfer” from the target MME (Step 11), the target base station receives “switching acknowledgment” information sent by the terminal (Step 12). After that, the target base station sends “switching notification” information to the target MME (Step 13). Immediately after that, the source MME and the target MME complete relocation (Step 14), and the source MME instructs the source base station to release resources (Step 15). Finally, a tracking area is updated, and switching is completed (Step 16). In an actual LTE switching process, the source base station further needs to forward data to the target base station to implement seamless switching.
In the cellular network switching in FIG. 2, the source base station selects a target base station to which the user terminal is handed over. However, in a D2D communication network under distributed control of a cluster head, the base station only has communication connection information of the cluster head, and does not know state information of other member terminals in the cluster, so the foregoing switching method is not applicable to seamless handover of a D2D cluster member from a D2D link to a cellular link.
In the prior art, most researches are based on D2D communication under centralized control of a base station. In a scenario of such centralized control, whether it is D2D pair or D2D cluster communication, the base station is in charge of managing the switch of the user terminal from D2D communication to cellular communication. However, for D2D cluster communication under distributed or semi-distributed control of a cluster head, except for the cluster head, other member terminals in the cluster lack dedicated channels connected to the base station; even if terminals belong to the same base station (for example, in FIG. 3, UE2 of the terminal and UE1 of the cluster head are located in a coverage area of the same base station), it is difficult to convert the terminals from D2D communication to cellular communication, and to convert terminals in different base stations (for example, in FIG. 3, UE2 moved to a coverage area of another base station and UE1 are located in coverage areas of different base stations) is even more difficult. Even if the switch between D2D communication and cellular communication can be implemented, it costs great signaling overhead and communication delay, which further decreases the system capacity and throughput and does harm to the advantage of D2D communication. Therefore, under the premise of reducing communication delay while still maintaining the quality of service (QoS) of the communication terminal, for the distributed cluster communication, mobile switching between D2D communication and cellular communication is an urgent problem to be solved.